Question

các bn tl giùm mik câu này nhé (cám ơn trc )

a, x/2008 - 1/10 -1/15 - 1/21 -...- 1/120 = 5/8

b, 7/x + 4/5.9 + 4/9.13 +...+ 4/41.45 = 29/45

C= 1/3.5 + 1/5.7 +...+ 1/2.(x+1).(2x+3) = 15/95

đề bài là tìm x thỏa mãn

bn nào tl đc mik tick liền

Answer 1

a: \(\Leftrightarrow\dfrac{x}{2008}-\dfrac{2}{20}-\dfrac{2}{30}-\dfrac{2}{42}-...-\dfrac{2}{240}=\dfrac{5}{8}\)

\(\Leftrightarrow\dfrac{x}{2008}-2\left(\dfrac{1}{20}+\dfrac{1}{30}+...+\dfrac{1}{240}\right)=\dfrac{5}{8}\)

\(\Leftrightarrow\dfrac{x}{2008}-2\left(\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+...+\dfrac{1}{15}-\dfrac{1}{16}\right)=\dfrac{5}{8}\)

\(\Leftrightarrow\dfrac{x}{2008}-2\left(\dfrac{1}{4}-\dfrac{1}{16}\right)=\dfrac{5}{8}\)

\(\Leftrightarrow\dfrac{x}{2008}-2\cdot\dfrac{3}{16}=\dfrac{5}{8}\)

=>x/2008=1

hay x=2008

b: \(\dfrac{7}{x}+\dfrac{4}{5\cdot9}+\dfrac{4}{9\cdot13}+...+\dfrac{4}{41\cdot45}=\dfrac{29}{45}\)

\(\Leftrightarrow\dfrac{7}{x}+\dfrac{1}{5}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{13}+...+\dfrac{1}{41}-\dfrac{1}{45}=\dfrac{29}{45}\)

\(\Leftrightarrow\dfrac{7}{x}+\dfrac{8}{45}=\dfrac{29}{45}\)

=>7/x=21/45=7/15

=>x=15